System for measuring earth formation resistivity through and electrically conductive wellbore casing

ABSTRACT

An apparatus is disclosed for measuring formation resistivity through a conductive pipe in a wellbore. The apparatus includes a sonde adapted to be moved through the wellbore, and a plurality of voltage measurement electrodes are disposed on the sonde at spaced apart locations. At least one current source electrode is disposed on the sonde. All the electrodes are adapted to make electrical contact with the pipe. The apparatus includes a digital voltage measuring circuit controllably coupled to selected ones of the voltage measurement electrodes.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF INVENTION

1. Field of the Invention

The invention relates generally to the field of Earth formationelectrical resistivity measuring devices. More particularly, theinvention relates to wellbore instruments for measuring formationresistivity from within an electrically conductive pipe or casing.

2. Background Art

Electrical resistivity measurements of Earth formations are known in theart for determining properties of the measured Earth formations.Properties of interest include the fluid content of the pore spaces ofthe Earth formations. Wellbore resistivity measuring devices known inthe art typically require that the Earth formations be exposed bydrilling a wellbore therethrough, and that such formations remainexposed to the wellbore so that the measurements may be made from withinthe exposed formations.

When wellbores are completely drilled through the Earth formations ofinterest, frequently a steel pipe or casing is inserted into andcemented in place within the wellbore to protect the Earth formations,to prevent hydraulic communication between subsurface Earth formations,and to provide mechanical integrity to the wellbore. Steel casing ishighly electrically conductive, and as a result makes it difficult touse conventional (so called “open hole”) techniques to determine theresistivity of the various Earth formations from within a steel pipe orcasing.

It is known in the art to make measurements for determining theelectrical resistivity of Earth formations from within conductive casingor pipe. A number of references disclose techniques for making suchmeasurements. A list of references which disclose various apparatus andmethods for determining resistivity of Earth formations from withinconductive casings includes: USSR inventor certificate no. 56052, filedby Alpin, L. M. (1939), entitled, The method for logging in cased wells;USSR inventor certificate no. 56026, filed by Alpin, L. M. (1939),entitled, Process of the electrical measurement of well casing; U.S.Pat. No. 2,459,196, to Stewart, W. H. (1949), entitled, Electricallogging method and apparatus; U.S. Pat. No. 2,729,784 issued to Fearon,R. E. (1956), entitled, Method and apparatus for electric well logging;U.S. Pat. No. 2,891,215 issued to Fearon, R. E. (1959), entitled, Methodand apparatus for electric well logging; French patent application no.72.41218, filed by Desbrandes, R. and Mengez, P. (1972), entitled,Method & Apparatus for measuring the formation electrical resistivity Inwells having metal casing; International Patent Application PublicationNo. WO 00/79307 A1, filed by Benimeli, D. (2002), entitled, A method andapparatus for determining of a formation surrounding a cased well; U.S.Pat. No. 4,796,186 issued to Kaufman, A. A. (1989), entitled,Conductivity determination in a formation having a cased well; U.S. Pat.No. 4,820,989, issued to Vail, III, W. (1989), entitled, Methods andapparatus for measurement of the resistivity of geological formationfrom within cased bore holes; U.S. Pat. No. 4,837,518 issued to Gard etal. (1989), entitled, Method and Apparatus for measuring the electricalresistivity of formation through metal drill pipe or casing; U.S. Pat.No. 4,882,542 issued to Vail, III, W. (1989), entitled, Methods andapparatus for measurement of electronic properties of geologicalformations through borehole casing; U.S. Pat. No. 5,043,668 issued toVail, III, W. (1991), entitled, Methods and apparatus for measurement ofelectronic properties of geological formations through borehole casing;U.S. Pat. No. 5,075,626 issued to Vail, III, W. (1991), entitled,Electronic measurement apparatus movable in a cased borehole andcompensation for casing resistance differences; U.S. Pat. No. 5,223,794issued to Vail, III, W. (1993), entitled, Methods of apparatus measuringformation resistivity from within a cased well having one measurementand two compensation steps; U.S. Pat. No. 5,510,712 issued to Sezgineret al. (1996), entitled, Method and apparatus for measuring formationresistivity in cased holes; U.S. Pat. No. 5,543,715 issued to Singer etal. (1996), entitled, Method and apparatus for measuring formationresistivity through casing using single-conductor electrical loggingcable; U.S. Pat. No. 5,563,514 issued to Moulin (1996), entitled, Methodand apparatus for determining formation resistivity in a cased wellusing three electrodes arranged in a Wheatstone bridge. U.S. Pat. No.5,654,639 issued to Locatelli et al. (1997), entitled, Inductionmeasuring device in the presence of metal walls; U.S. Pat. No. 5,570,024issued to Vail, III, W. (1996), entitled, Determining resistivity of aformation adjacent to a borehole having casing using multiple electrodesand resistances being defined between the electrodes; U.S. Pat. No.5,608,323 issued to Koelman, J. M. V. A. (1997), entitled, Arrangementof the electrodes for an electrical logging system for determining theelectrical resistivity of subsurface formation; U.S. Pat. No. 5,633,590issued to Vail, III, W. (1997), entitled, Formation resistivitymeasurements from within a cased well used to quantitatively determinethe amount of oil and gas present. U.S. Pat. No. 5,680,049 issued toGissler et al. (1997), entitled, Apparatus for measuring formationresistivity through casing having a coaxial tubing inserted therein;U.S. Pat. No. 5,809,458 issued to Tamarchenko (1998), entitled, Methodof simulating the response of a through-casing resistivity well logginginstrument and its application to determining resistivity of earthformations; U.S. Pat. No. 6,025,721 issued to Vail, III, W. (2000),entitled, Determining resistivity of a formation adjacent to a boreholehaving casing by generating constant current flow in portion of casingand using at least two voltage measurement electrodes; U.S. Pat. No.6,157,195 issued to Vail, III, W. (2000), entitled, Formationresistivity measurements from within a cased well used to quantitativelydetermine the amount of oil and gas present; U.S. Pat. No. 6,246,240 B1issued to Vail, III, W. (2001), entitled, Determining resistivity offormation adjacent to a borehole having casing with an apparatus havingall current conducting electrodes within the cased well; U.S. Pat. No.6,603,314 issued to Kostelnicek et al. (2003), entitled, Simultaneouscurrent injection for measurement of formation resistance throughcasing; and U.S. Pat. No. 6,667,621 issued to Benimelli, entitled,Method and apparatus for determining the resistivity of a formationsurrounding a cased well.

United States Patent Application Publications which cite relevant artinclude No. 2001/0033164 A1, filed by Vinegar et al., entitled, Focusedthrough-casing resistivity measurement; No. 2001/0038287 A1, filed byAmini, Bijan K., entitled, Logging tool for measurement of resistivitythrough casing using metallic transparencies and magnetic lensing; No.2002/0105333 A1 filed by Amini, Bijan K., entitled, Measurements ofelectrical properties through non magnetically permeable metals usingdirected magnetic beams and magnetic lenses. and No. 2003/0042016 A1,filed by Vinegar et al., entitled, Wireless communication using wellcasing.

The foregoing techniques are summarized briefly below. U.S. Pat. No.2,459,196 describes a method for measuring inside a cased wellbore,whereby electrical current is caused to flow along the conductive casingsuch that some of the current will “leak” into the surrounding Earthformations. The amount of current leakage is related to the electricalconductivity of the Earth formations. The '196 patent does not discloseany technique for correcting the measurements for electricalinhomogeneities in the casing.

U.S. Pat. No. 2,729,784 discloses a technique in which three potentialelectrodes are used to create two opposed pairs of electrodes in contactwith a wellbore casing. Electrical current is caused to flow in twoopposing “loops” through two pairs of current electrodes placed aboveand below the potential electrodes such that electrical inhomogeneitiesin the casing have their effect nulled. Voltage drop across the twoelectrode pairs is related to the leakage current into the Earthformations. The disclosure in U.S. Pat. No. 2,891,215 includes a currentemitter electrode disposed between the measuring electrodes of theapparatus disclosed in the '784 patent to provide a technique for fullycompensating the leakage current.

U.S. Pat. No. 4,796,186 discloses the technique most frequently used todetermine resistivity through conductive casing, and includes measuringleakage current into the Earth formations, and discloses measuringcurrent flowing along the same portion of casing in which the leakagecurrent is measured so as to compensate the measurements of leakagecurrent for changes in resistance along the casing. Other referencesdescribe various extensions and improvements to the basic techniques ofresistivity measurement through casing.

The methods known in the art for measuring resistivity through casingcan be summarized as follows. An instrument is lowered into the wellborehaving at least one electrode on the instrument (A) which is placed intocontact with the casing at various depths in the casing. A casingcurrent return electrode B is disposed at the top of and connected tothe casing. A formation current return electrode B* is disposed at theEarth's surface at some distance from the wellbore. A record is made ofthe voltage drop and current flowing from electrode A in the wellbore atvarious depths, first to electrode B at the top of the casing and thento formation return electrode B*. Current flow and voltage drop throughthe casing (A-B) is used to correct measurements of voltage drop andcurrent flow through the formation (A-B*) for effects of inhomogeneityin the casing.

If the Earth and the casing were both homogeneous, a record with respectto depth of the voltage drop along the casing, and the voltage dropthrough the casing and formation, would be substantially linear. As iswell known in the art, casing includes inhomogeneities, even when new,resulting from construction tolerances, composition tolerances, and even“collars” (threaded couplings) used to connect segments of the casing toeach other. Earth formations, of course, are not at all homogeneous, andmore resistive formations are typically the object of subsurfaceinvestigation, because these Earth formations tend to be associated withpresence of petroleum, while the more conductive formations tend to beassociated with the presence of all connate water in the pore spaces.Therefore, it is the perturbations in the record of voltage drop withrespect to depth that are of interest in determining resistivity ofEarth formations outside casing using the techniques known in the art.

The conductivity of the Earth formations is related to the amount ofcurrent leaking out of the casing into the formations. The formationconductivity with respect to depth is generally related to the secondderivative of the voltage drop along A-B with respect to depth, whencurrent is flowing between A and B*. Typically, the second derivative ofthe voltage drop is measured using a minimum of three axially spacedapart electrodes placed in contact with the casing, coupled to cascadeddifferential amplifiers, ultimately coupled to a voltage measuringcircuit. Improvements to the basic method that have proven usefulinclude systems which create s small axial zone along the casing inwhich substantially no current flows along the casing itself to reducethe effects of casing inhomogeneity on the measurements of leakagecurrent voltage drop.

In practice, instruments and methods known in the art require that theinstrument make its measurements from a fixed position within thewellbore, which makes measuring formations of interest penetrated by atypical wellbore take an extensive amount of time. Further, the voltagedrops being measured are small, and thus subject to noise limitations ofthe electronic systems used to make the measurements of voltage drop.Still further, systems known in the art for providing no-current zones,or known current flow values for measurements of voltage drop, aretypically analog systems, and thus subject to the accuracy limitationsof such analog systems.

Still further, it is known in the art to use low frequency alternatingcurrent (AC) to induce current flow along the casing and in the Earthformations. AC is used to avoid error resulting from electricalpolarization of the casing and the electrodes when continuous directcurrent (DC) is used. Typically, the frequency of the AC must be limitedto about 0.01 to 20 Hz to avoid error in the measurements caused bydielectric effects and the skin effect. It is also known in the art touse polarity-switched DC to make through casing resistivitymeasurements, which avoids the polarization problem, but may inducetransient effect error in the measurements when the DC polarity isswitched. Transient effects, and low frequency AC errors are not easilyaccounted for using systems known in the art.

SUMMARY OF THE INVENTION

One aspect of the invention is an apparatus for measuring formationresistivity through a conductive pipe in a wellbore. The apparatusincludes a sonde adapted to be moved through the wellbore, and aplurality of potential measurement electrodes are disposed on the sondeat spaced apart locations. At least one current source electrode isdisposed on the sonde. All the electrodes are adapted to make electricalcontact with the pipe. The apparatus includes a digital voltagemeasuring circuit controllably coupled to selected ones of the potentialmeasurement electrodes.

Another aspect of the invention is a method for measuring resistivity ofEarth formations from within a conductive pipe inside a wellbore. Amethod according to this aspect of the invention includes conducting anelectrical current between a first selected position in the wellborethrough the conductive pipe to a second position along the pipe near theEarth's surface. A voltage drop measured between a third and fourthselected positions along the pipe between the first and second selectedpositions is digitally sampled. An electrical current is then conductedbetween the first selected position and a fifth selected position nearthe Earth's surface away from the pipe. The digitally sampling thevoltage drop between the third and fourth positions is repeated.Resistivity of the Earth formation is determined from the digitalsamples of voltage drop.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example resistivity measurement through casing apparatusaccording to the invention being used in a cased wellbore.

FIG. 2 shows a circuit systems of the example apparatus of FIG. 1 inmore detail.

FIGS. 3A through 3C show different examples of current waveform formaking through casing resistivity measurements according to theinvention.

FIG. 4 shows an example instrument for measuring resistivity through aconductive pipe which includes current focusing systems.

FIG. 5 shows an alternative embodiment of an apparatus including aselectable array of electrodes on a sonde mandrel.

FIG. 6 shows a flow chart of operation of an instrument such as shown inFIG. 4 adapted to automatically optimize control of electrode usageaccording to a model based instrument response.

DETAILED DESCRIPTION

One embodiment of a well logging instrument used to measure resistivityof Earth formations from within a wellbore, when the wellbore has aconductive pipe or casing within is shown schematically in FIG. 1. Theinstrument 10 may include a sonde or similar mandrel-type housing 18.The housing 18 is preferably made from an electrically non-conductivematerial, or has such non-conductive material on its exterior surface.The housing 18 is adapted to be inserted into and withdrawn from thewellbore 14, by means of any well logging instrument conveyance known inthe art. In the present example, the conveyance can be an armoredelectrical cable 16 extended and retracted by a winch 28. Otherconveyances known in the art may be used, including coiled tubing, drillpipe, production tubing, etc. Accordingly, the conveyance is notintended to limit the scope of the invention.

The wellbore 14 is drilled through various Earth formations, shownschematically at 22, 24 and 26. After the wellbore 14 is drilled, aconductive pipe 12 or casing is inserted into the wellbore 14. If thepipe 12 is a casing, then the casing 12 is typically cemented in placewithin the wellbore 14, although cementing the pipe or casing is notnecessary to operation of the instrument 10. While the embodiment shownin FIG. 1 is described in terms of a “casing” being inserted andcemented into a drilled wellbore, it should be understood that othertypes of electrically conductive pipe, such as drill pipe, coiledtubing, production tubing and the like may also be used with aninstrument according to the invention. In one particular example, thepipe 12, rather than being casing, may be drill pipe that has becomestuck in the wellbore 14, whereupon the instrument 10 is lowered intothe stuck drill pipe on an armored electrical cable 16 to makemeasurements as will be further explained.

The armored electrical cable 16 includes one or more insulatedelectrical conductors (not shown separately) and is arranged to conductelectrical power to the instrument 10 disposed in the wellbore 14.Electrical power can be conducted from, and signals from the instrument10 can be transmitted to, a recording unit 30 disposed at the Earth'ssurface using the electrical conductors on the cable 16. The recordingunit 30 may also be used to record and/or interpret the signalscommunicated thereto from the instrument 10 in the wellbore 14. Therecording unit 30 may include an electrical power supply 32 used to makemeasurements for determining resistivity of the various Earth formations22, 24, 26. In the present description, any electrical power supply usedto enable making the measurements corresponding to formation resistivitywill be referred to as a “measuring current source.” The power supply 32may also be used merely to provide electrical power to variousmeasurement and control circuits, shown generally at 20 in FIG. 1, inthe instrument 10. The functions provided by the various circuits in theinstrument will be further explained below with reference to FIG. 2.

Still referring to FIG. 1, a measuring current return electrode 34B* isprovided at the Earth's surface at a selected distance from the wellbore14. The measuring current return electrode 34B* is typically insertedinto formations proximate the Earth's surface so as to provide anelectrically conductive path to the Earth formations 22, 24, 26penetrated by the wellbore 14. The measuring current return electrode34B* provides, in particular, a current path through the Earthformations 22, 24 26 for electrical measuring current to flow from asource electrode A on the instrument 10. The current return electrode34B* may be connected, as shown in FIG. 1, either to circuits 35B* inthe recording unit 30, or alternatively may be connected to one of theelectrical conductors (not shown separately) in the cable 16. A casingcurrent return electrode 34B, shown connected to the top of the pipe orcasing 12, provides a return path for electrical measuring currentcaused to flow from the current source electrode A on the instrument 10,to the top of the casing 12. The casing current return electrode 34B maybe coupled to circuits 35B in the recording unit 30, or may be coupledto one of the conductors (not shown) in the cable 12 for return to thecircuits 20 in the instrument 10.

The instrument 10 includes a plurality of electrodes, shown at A, and P0through P6 disposed on the sonde mandrel 18 at axially spaced apartlocations. The electrodes A, P0-P6 are electrically isolated from eachother by the non-conductive material disposed on the exterior of, orforming, the sonde mandrel 18. Each of the electrodes A, P0-P6 ismechanically and electrically adapted to make good electrical contactwith the casing 12. Various types of casing-contact electrodes are knownin the art and include brushes, hydraulically actuated “spikes”, spikedwheels and similar devices. The electrodes A, P0-P6 are each coupled toa selected portion of the electronic circuits 20 in the instrument 10.

During operation of the instrument 10 when conveyed by armored cable,the cable 16 is extended by the winch 28 so that the instrument 10 ispositioned at a selected depth in the wellbore 14. Electrical power ispassed through the casing 12 and through the Earth formations 22, 24, 26by selective connection between the source electrode A at one end of thecurrent path, and either the casing return 34B or formation return 34B*,respectively, at the other end of the current path. Measurements aremade of the voltage extant between a reference potential electrode,shown as electrode P0 in FIG. 1, and one or more potential measurementelectrodes, P1-P6 in FIG. 1. Depending on the type of electrodes used,for example, brushes or spiked contact wheels, it may be possible, insome embodiments, for the instrument 10 to be moved slowly along thewellbore 14 as the measurements are being made. Other types ofelectrode, such as hydraulically actuated spikes, may require that theinstrument 10 remain essentially stationary during any one measurementsequence. As the voltage measurements are made, whether the instrument10 is stationary or moving, the instrument 10 is gradually withdrawnfrom the wellbore 14, until a selected portion of the wellbore 14,including formations of interest, 22, 24, 26, have voltage measurementsmade corresponding to them, both using the casing current return 34B andthe formation current return 34B*.

One embodiment of the electronic circuits 20 is shown in greater detailin FIG. 2. The present embodiment of the circuits 20 may include acentral processing unit (CPU) 50, which may be a preprogrammedmicrocomputer, or a programmable microcomputer. In the presentembodiment, the CPU 50 is adapted to detect control commands from withina formatted telemetry signal sent by the recording unit (30 in FIG. 1)to a telemetry transceiver and power supply unit 48. The telemetrytransceiver 48 also performs both formatting of data signalscommunicated by the CPU 50 for transmission along a cable conductor 16Ato the recording unit (30 in FIG. 1) and reception and conditioning ofelectrical power sent along the conductor 16A for use by the variouscomponents of the circuits 20. The CPU 50 may also be reprogrammed bythe command signals when such are detected by the telemetry transceiver48 and conducted to the CPU 50. Reprogramming may include, for example,changing the waveform of the measure current used to make the previouslyexplained voltage drop measurements. Reprogramming may also includechanging the magnitude of the measure current, and may include changinga sample rate of voltage drop measurements, among other examples. Stillother forms of reprogramming will be explained with reference to FIGS. 4through 6.

While the embodiment shown in FIG. 2 includes an electrical telemetrytransceiver 48, it should be clearly understood that optical telemetrymay be used in some embodiments, and in such embodiments the telemetrytransceiver 48 would include suitable photoelectric sensors and/ortransmitting devices known in the art. In such embodiments, the cable 16should include at least one optical fiber for conducting such telemetrysignals. One embodiment of an armored electrical cable including opticalfibers therein for signal telemetry is disclosed in U.S. Pat. No.5,495,547 issued to Rafie et al. Other embodiments may use opticalfibers to transmit electrical operating power to the instrument 10 fromthe recording unit 30. The cable disclosed in the Rafie et al. '547patent or a similar fiber optic cable may be used in such otherembodiments to transmit power to the instrument over optical fibers.

The CPU 50 may include in its initial programming (or may be soprogramed by reprogramming telemetry signals) a digital representationof various current waveforms used to energize the Earth formations (22,24 26 in FIG. 1) and the casing (12 in FIG. 1) for determining theresistivity of the Earth formations (22, 24, 26 in FIG. 1). The digitalrepresentation includes information about the frequency content, theshape of the waveform and the amplitude of the current to be conductedthrough the formations and casing. The digital representation can beconducted to a digital to analog converter (DAC) 42, which generates ananalog signal from the digital representation. The analog signal outputof the DAC 42 is then conducted to the input of a power amplifier 44.The power amplifier 44 output is connected between the current sourceelectrode A and a switch 47. The switch 47 is under control of the CPU50. The switch 47 alternates connection of the other output terminal ofthe power amplifier 44 between the casing return electrode B and theformation return electrode B*, or other current electrodes in otherelectrode arrangements. Alternatively, the other output terminal of thepower amplifier 44 may be connected to one of more cable conductors(either 16A or other electrical conductor), and the switching betweencasing return and formation return may be performed within the recordingunit (30 in FIG. 1). Yet another alternative omits the DAC 42 and thepower amplifier 44 from the circuits 20, and provides measuring currentand switching features using the power supply (32 in FIG. 1) in therecording unit (30 in FIG. 1) and appropriate conductors (not shown) inthe cable (16 in FIG. 1). In the latter example embodiment, measuringcurrent may be conducted to the source electrode A using one or morecable conductors, such as 16A in FIG. 2.

In the present embodiment, voltage measurements can be made between thepotential reference electrode P0 and a selected one of the potentialmeasuring electrodes P1-P6. The one of the voltage measuring electrodesfrom which measurements are made at any moment in time can be controlledby a multiplexer (MUX) 40, which itself may be controlled by the CPU 50.The output of the MUX 40 is connected to the input of a low noisepreamplifier or amplifier 38. The output of the preamplifier 38 iscoupled to an analog to digital converter (ADC) 36. The ADC 36 may be asigma delta converter, successive approximation register, or any otheranalog to digital conversion device known in the art, that preferablycan provide at least 24 bit resolution of the input signal. Digitalsignals output from the ADC 36 represent the measured potential betweenthe reference electrode P0 and the MUX-selected one of the voltagemeasuring electrodes P1-P6. One possible advantage of using the MUX 40and single preamplifier 38 as shown in FIG. 2 is that the analog portionof the voltage measuring circuitry will be substantially the sameirrespective of which voltage measuring electrode P1-P6 is beinginterrogated to determine potential drop with respect to electrode P0.As a result, measurement error caused by differences in preamplifier 38response may be reduced or eliminated. Preferably, the ADC 36 is atwenty-four bit device capable of accurately resolving measurementsrepresenting voltage differences as small as one nanovolt (1×10⁻⁹volts). Alternatively, each measurement electrode P1-P6 could be coupledto one input terminal of a separate preamplifier (not shown in theFigures) for each electrode P1-P6, thus eliminating the MUX 40 from theanalog input circuitry.

Digital words representing the voltage measurements can be conductedfrom the ADC 36 to the CPU 50 for inclusion in the telemetry to therecording unit (30 in FIG. 1). Alternatively, the CPU 50 may include itsown memory or other storage device (not shown separately) for storingthe digital words until the instrument (10 in FIG. 1) is removed fromthe wellbore (14 in FIG. 1). In some embodiments, a sample rate of theADC 36 is in the range of several kilohertz (kHz) both to provide both avery large number of voltage signal samples, preferably at least onethousand, per cycle of current waveform, and to be able to sampletransient effects when switched DC is used as a current source to makeresistivity measurements. In such embodiments, a switching frequency ofthe switched DC can be in a range of about 0.01 to 20 Hz, thus enablingthe ADC 36 to make preferably at least one thousand, and as many asseveral thousand, voltage measurement samples within each cycle of theswitched DC.

In the present embodiment, the ADC 36 operates substantiallycontinuously, to provide a relatively large number of digital signalsamples for each cycle of the current source waveform. In the presentembodiment, such substantially continuous operation of the ADC 36 mayprovide the advantage of precise, prompt determination of any DC bias inthe voltage measurements. Such DC bias must be accounted for in order toprecisely determine formation resistivity from the voltage measurements.In systems known in the art which do not operate voltage measuringdevices substantially continuously, it is necessary to determine DC biasby other means. See, for example, U.S. Pat. No. 5,467,018 issued toRueter et al.

The measuring current waveform, as previously explained, may begenerated by conducting waveform numerical values from the CPU 50, orother storage device (not shown) to the DAC 42. Referring now to FIGS.3A through 3C, several types of current waveforms particularly suited tomaking through-casing (or through electrically conductive pipe)resistivity measurements will be explained. FIG. 3A is a graph ofcurrent output of the power amplifier (44 in FIG. 2) with respect totime. The current waveform 60 in FIG. 3A is a low frequency (0.01 to 20Hz) square wave, which may be generated using switched DC, or byconducting appropriate numbers representing such a waveform to the DAC(42 in FIG. 2). The waveform 60 in FIG. 3A is periodic, meaning that thewaveform is substantially constant frequency within a selected timerange, and has 100 percent “duty cycle”, meaning that current is flowingsubstantially at all times.

Another possible current waveform is shown at 60 in FIG. 3B. The currentwaveform in FIG. 3B is a random or pseudo random frequency square wave,also having 100 percent duty cycle. As with the previous embodiment(FIG. 3A), the embodiment of current waveform shown in FIG. 3B may begenerated by conducting appropriate digital words from the CPU (50 inFIG. 2) to the DAC (42 in FIG. 2). Random switching will be advantageousto avoid aliasing or other adverse effects related to periodic datasampling.

Another possible waveform is shown at 60 in FIG. 3C. The currentwaveform 60 in FIG. 3C is a periodic square wave having less than 100percent duty cycle. Less than 100 percent duty cycle can be inferredfrom time intervals, shown at 62, in which no current is flowing. Aswith the previous embodiment (FIG. 3A), the embodiment of currentwaveform shown in FIG. 3C may be generated by conducting appropriatedigital words from the CPU (50 in FIG. 2) to the DAC (42 in FIG. 2).Using less than 100 percent duty cycle may be advantageous to saveelectrical power where measured voltage drops are sufficiently large tomake possible a reduction in the number of voltage samples measured.Using less than 100 percent duty cycle may also enable determination ofsome transient effects, by measuring voltage drops across the variouselectrodes (P0 b between P1-P6 in FIG. 1) during a short time intervalafter the current is switched off. Such induced potential (IP) effectsmay be related to fluid composition within the pore spaces of the Earthformations (22, 24, 26 in FIG. 1). Using less than 100 percent dutycycle may also enable better determination of any DC bias, by using thetimes with no current flow 62 as measurement references.

The foregoing examples shown in FIGS. 3A, 3B and 3C are not the onlycurrent waveforms that may be generated using the CPU/DAC combinationshown in FIG. 2. As will be readily appreciated by those skilled in theart, substantially any frequency and waveform type may be generated,including for example sinusoidal waveforms, by conducting appropriatedigital words to the DAC (42 in FIG. 2). In some embodiments, thedigital words may be stored in the CPU (50 in FIG. 2). In otherembodiments, the digital words themselves, or a command which activatesselected waveform digital words, may be transmitted from the recordingunit (30 in FIG. 1) to the instrument (10 in FIG. 1) over the cable (16in FIG. 1). In other embodiments, the waveform may be a pseudo randombinary sequence (PRBS).

Referring once again to FIG. 2, some embodiments may include one or moreof the following features, either programmed into the CPU 50, orprogrammed into a surface computer in the recording unit (30 in FIG. 1).Some embodiments may include automatic editing of voltage measurementsmade across the one or more electrode pairs, P0 between any one ofP1-P6. For example, if a particular digital voltage sample represents anumber outside of a selected range, the sample may be discarded, and aninterpolated value may be written to storage in the CPU 50, ortransmitted to the recording unit (30 in FIG. 1) for the outlying samplevalue. Alternatively, if voltage measurements do not increasemonotonically as the spacing between P0 and the various measurementelectrodes P1-P6 is increased, the anomalous voltage samples may bediscarded; interpolated or otherwise not written directly to storage.Other embodiments may include stacking of voltage measurement wordscorresponding to the same electrode pair (P0 between any of P1-P6) atsubstantially the same depth in the wellbore to improve the signal tonoise ratio of the measurements significantly.

Referring once again to FIG. 1, still other embodiments may includepermanent installation of an array of electrodes, such as shown in FIG.1 at A and P0 through P6 inside the casing 16. A cable or similar devicemay be used to make electrical connection to the Earth's surface frominside the wellbore 14 at a selected depth proximate a petroleum bearingreservoir, for example, formation 24 in FIG. 1. Measurements may be madeat selected times during the life of the wellbore 14 to determinemovement of a water contact (not shown in FIG. 1) with respect to time.In such permanent emplacements of electrodes A, P0-P6, the circuits 20may be disposed at the Earth's surface, or may themselves be disposed inthe wellbore 14, just as for the cable conveyed instrument describedearlier herein.

Operating the instrument may be performed in a number of different ways,of which several will be explained herein. In a regular measurementmode, the instrument 10 may be moved to a selected depth in the wellbore14 at which measurements are to be made. First, the circuits 20 areoperated, either by internal programming of the CPU (50 in FIG. 2) or bycommand transmitted from the recording unit (30 in FIG. 1) first toenable measuring voltage drop caused by current flow entirely along thecasing 12. To make casing voltage drop measurements, the power amplifier(44 in FIG. 2) is connected between the current source electrode A onthe instrument 10 and casing current return electrode 34B coupled to thetop of the casing (12 in FIG. 1) at the Earth's surface. Voltagemeasurements between P0 and any one or more of P1 through P6 are thenmade. The output of the power amplifier (44 in FIG. 2) is then switchedto return the measuring current at measuring current return electrode34B* at the Earth's surface. Another set of voltage measurements betweenP0 and the same ones of P1 through P6 are made. The instrument 10 maythen be moved a selected axial distance along the wellbore 14, and themeasuring process can be repeated. Values of voltage difference madebetween P0 and any one or more of P1 through P6 can be convertedmathematically into a second derivative, with respect to depth in thewellbore 14, of the measured voltage drop. The values of such secondderivative are related to the depth-based current leakage into the Earthformations 22, 24, 26, and are thus related to the electricalconductivity of each of the formations 22, 24, 26. Advantageously, aninstrument configured substantially as shown in FIGS. 1 and 2 does notrequire measurement of voltage drop across cascaded differentialamplifiers (all of which would be analog) to determine the secondderivative of voltage drop with respect to depth.

Performance of an instrument according to the invention may be improvedby providing focusing current systems to axially constrain the flow ofmeasuring current through the various Earth formations. An exampleinstrument which includes focusing current systems is shownschematically in FIG. 4. The principle of measurement of the exampleinstrument shown in FIG. 4 is described in U.S. Pat. No. 2,729,784issued to Fearon, incorporated herein by reference. The instrument inFIG. 4 includes an array of electrodes disposed at selected locationsalong the instrument mandrel or housing (18 in FIG. 1). The electrodesmay be similar in mechanical and electrical configuration to theelectrodes described above with reference to FIG. 1. The electrodes areadapted to make electrical contact with the pipe or casing (12 inFIG. 1) in the wellbore (14 in FIG. 1).

The electrodes in the embodiment of FIG. 4 include two pairs of focusingcurrent electrodes, shown at B1A, B1B and B2A, B2B, approximatelyequally spaced on either axial side of a central measuring currentsource electrode M0. Reference potential measuring electrodes R1A, R1Band R2A, R2B are disposed, respectively, between each focusing currentelectrode pair B1A, B1B; B2A, B2B, and the measuring current sourceelectrode M0. Each focusing current electrode pair B1A, B1B and B2A, B2Bis connected across the output of a corresponding focusing current poweramplifier 44A, 44C, respectively. In the present embodiment, thefocusing current is generated by driving each power amplifier 44A, 44Cusing the output of a corresponding DAC 42A, 42C. Each DAC 42A, 42C canbe connected to a bus or other similar data connection to the CPU 50. Asin the embodiment explained above with reference to FIG. 2, theembodiment shown in FIG. 4 may include digital words stored orinterpreted by the CPU 50 which represent the focusing current waveformto be generated by each power amplifier 44A, 44C and conducted to thecasing (12 in FIG. 1). Aspects of the waveform which may be controlledinclude amplitude, phase, frequency and duty cycle, among other aspects.

Each pair of reference potential measuring electrodes R1A, R1B and R2A,R2B is coupled across the input terminals of a respective low noisepreamplifier 38A, 38B, or low noise amplifier, similar to thepreamplifier described with reference to FIG. 2. Each low noisepreamplifier 38A, 38B has its output coupled to an ADC 42A, 42B. The ADC42A, 42B outputs are coupled to the bus or otherwise to the CPU 50. Inthe present embodiments, the ADCs 42A, 42B are preferably 24 bitresolution devices, similar to the ADC described with reference to FIG.2. In the present embodiment, potential difference measurements are madeacross each pair of reference potential electrodes R1A, R1B and R2A,R2B, respectively. The CPU 50 receives digital words representing themeasured potential across each reference electrode pair R1A, R1B andR2A, R2B, respectively. The magnitude of the focusing current output byeach power amplifier 44A, 44C can be controlled by the CPU 50 such thatthe measured potential across each pair of reference potentialelectrodes R1A, R1B and R2A, R2B, respectively, is substantially equalto zero. The CPU 50 may cause such adjustments to be made by, forexample, changing the amplitude or changing the duty cycle of the poweramplifier 44A, 44B outputs, or both. Changes to amplitude and/or dutycycle may be made to either or both power amplifier 44A, 44B. Othermethods for changing or adjusting the power output of each focusingcurrent power amplifier 44A, 44C will occur to those skilled in the art.The purpose of making such focusing current magnitude adjustments so asto maintain substantially zero potential across the reference electrodesR1A, R1B and R2A, R2B, respectively, is to assure that there is a regionwithin the casing (12 in FIG. 1) where substantially no net currentflows along the casing in either an upward or downward direction.

The embodiment of FIG. 4 can include a digitally controlled measuringcurrent source. The source consists of, in the present embodiment, ameasuring current DAC 42B coupled to the bus or otherwise to the CPU 50.Measuring current is generated by conducting waveform words to the DAC42B, which converts the words into a driver signal for a measuringcurrent power amplifier 44B coupled at its input to the DAC 42B output.Measuring current output from the measuring current power amplifier 44Bis coupled to the measuring current source electrode M0, and maybereturned at the Earth's surface, at return electrode 34B*, oralternatively at casing current return 34B. Measuring potentialelectrodes M1A, M1B are disposed on either side of the measuring currentsource electrode M0. Each measuring potential electrode M1A, M1B, andthe source electrode M0 is coupled across the input of a respectivemeasuring potential low noise amplifier 38B, 38C. The output of eachmeasuring potential low noise amplifier 38B, 38C is coupled to arespective ADC 36B, 36C, wherein digital words representing the value ofmeasured potential across each respective pair of measure potentialelectrodes M1A, M0 and M1B, M0 are conducted to the CPU 50 forprocessing. The measuring potential ADC 44B is also preferably a 24 bitresolution device. Resistivity of the Earth formations outside thecasing is related to the potential across the measuring potentialelectrodes and the magnitude of the measuring current. Waveform,frequency and duty cycle of the measuring current may be controlled in asubstantially similar manner as explained with reference to theembodiment of FIG. 2.

Possible advantages of a system as shown in FIG. 4 include more accuratecontrol over focusing current properties than was previously possible,making measurements of potential across the measuring electrodes M1A,M1B more accurate.

Another embodiment of an instrument according to the invention is shownschematically in FIG. 5. The instrument includes an array of electrodesdisposed on the instrument housing 18 at axially spaced apart locations.The electrodes are designated A, B, P, O, N and M. The electrodes arecoupled through a switching system, designated “control unit” 50A (whichmay be associated with for form part of a controller similar in designto CPU 50 from FIG. 2). The control unit 50A selects which electrodesare coupled to which one or selected circuits. The circuits include acurrent source 52. The current source 52 may be a digital synthesizer,and may include a DAC and power amplifier (not shown separately). Thecircuits may include a voltage (or potential) measuring circuit 51,which may include a low noise preamplifier and ADC (not shownseparately) as explained with reference to FIG. 2. The circuits may alsoinclude a voltage feedback unit 53, which may be similar inconfiguration to the focusing current source explained with reference toFIG. 4.

To perform various types of measurements, the instrument shown in FIG. 5can select the measuring and focusing current sources to be applied to,and voltage measurements to be made across, selected ones of theelectrodes and selected electrode pairs. Examples of various modes ofmeasurement, and the electrodes used to make measurements in each of themodes, are explained in the following table: Current source andPotential measured Measurement Mode return electrodes across electrodesDownhole, completely A, B M and N; O and P contained Deep penetratingresistivity B, current return is at M and N; O and P Earth's surfaceaway from top of casing (return 34B*) Fast measurement M and N A and B;O and P Mixed Mix sources Mix pairs

In the above table, the “Current source and return electrodes” columnrepresents the electrodes coupled to the measuring current source 52.Potential measurement is made across electrode pairs as indicated in the“Potential measured across electrodes” column.

Various configurations of an instrument according to the invention whichinclude a suitably programmed CPU (50 in FIG. 2) may providesubstantially real-time automatic control of selection of the variouselectrodes for the purposes as explained above with reference to FIG. 4,namely axial spacings of the voltage measuring electrodes, and thespacing of and amount of focusing current supplied to various focusingelectrodes. A generalized flow chart showing one embodiment of a systemprogrammed to perform the foregoing functions is shown in FIG. 6. At 70,initially configured electrodes, current sources and voltage measuringcircuits emit measuring current, focusing current and make voltagemeasurements, respectively. Initial configuration may be set by thesystem operator, or may be preprogrammed. Preprogrammed oroperator-selected initial configuration may be based on parameters suchas expected thickness of the various Earth formations and expectedresistivities of the various Earth formations, among other parameters.At 71, voltages are measured, at least for one pair of voltage measuringelectrodes. In configurations which include reference potentialelectrodes, for example as explained with reference to FIG. 4, suchreference potentials may also be measured. At 72 the measured voltagesare analyzed. Analysis may include determining a magnitude of voltagedrop along the casing to determine casing resistance, and may includedetermining voltage drop of leakage current into the formations.Analysis may include determination of polarization direction forreference potential measurements which are not substantially equal tozero. At 75, the analysis is used to determine if the response obtainedrepresents a stable set of formation resistivity calculations. If theresponse is stable, at 77, the voltage measurements are used todetermine formation resistivity, typically, as previously explained, bydetermining a second derivative, with respect to depth, of the magnitudeof leakage current corrected for casing resistance variation in thevicinity of where the measurements are made.

At 73, the voltage measurements may be used to develop a model of theresistivity distribution around the outside of the wellbore (14 inFIG. 1) proximate the instrument (10 in FIG. 1). Methods for determininga model of the Earth formations are disclosed, for example, in U.S. Pat.No. 5,809,458 issued to Tamarchenko (1998), entitled, Method ofsimulating the response of a through-casing resistivity well logginginstrument and its application to determining resistivity of earthformations. At 74, the model is subjected to a sensitivity analysis. Themodel, using appropriate sensitivity analysis, may be used, at 76, todetermine an optimum arrangement of focusing current electrodes. If thedetermined optimum focusing current electrode arrangement is differentfrom the initial or current configuration, the configuration is changed,at 79, and focusing current parameters are changed at 78 to provide themodel with the optimum sensitivity response.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. An apparatus for measuring formation resistivity through a conductivepipe in a wellbore, comprising: a sonde adapted to be moved through thewellbore; a plurality of voltage measurement electrodes disposed on thesonde at spaced apart locations, the voltage electrodes adapted to makeelectrical contact with the pipe; at least one current source electrodedisposed on the sonde, the current source electrode adapted to makecontact with the pipe; a digital voltage measuring circuit controllablycoupled to selected ones of the voltage measurement electrodes; and acurrent return electrode coupled to the pipe proximate the Earth'ssurface and a current return electrode disposed proximate the Earth'ssurface at a selected lateral distance from the current return electrodecoupled to the pipe, and a switch to select a return path for measuringcurrent from the current source electrode to the selected one of theelectrode coupled to the top of the pipe and the electrode disposed atthe selected lateral distance from the pipe.
 2. The apparatus of claim 1wherein the digital voltage measuring circuit comprises of at least atwenty four bit resolution analog to digital converter.
 3. The apparatusof claim 2 wherein the analog to digital converter has a sampling rateof at least one thousand times a frequency of electrical current used toenergize the at least one current source electrode.
 4. The apparatus ofclaim 1 further comprising a digitally synthesized current sourcecoupled to the at least one current source electrode.
 5. The apparatusof claim 4 wherein the current source is adapted to generate switcheddirect current.
 6. The apparatus of claim 4 wherein the current sourceis adapted to generate switched direct current having less than a onehundred percent duty cycle.
 7. The apparatus of claim 4 wherein thecurrent source is adapted to generate alternating current having aselected frequency and waveform.
 8. The apparatus of claim 4 wherein thecurrent source is adapted to generate a pseudo random binary sequence.9. The apparatus of claim 1 further comprising at least one focusingcurrent source controllable to maintain a selected voltage drop across apair of reference potential electrodes, the focusing current sourceelectrically coupled to selected electrodes on the sonde.
 10. Theapparatus of claim 1 wherein the digital voltage measuring circuit isadapted to determine a direct current bias extant on the voltagemeasurement electrodes by operating substantially continuously.
 11. Theapparatus of claim 1 further comprising: at least one focusing currentsource controllable to maintain a selected voltage drop across a pair ofreference potential electrodes, the focusing current source electricallycoupled to selected electrodes on the sonde; and a switch adapted toselectively connect selected ones of the electrodes to the focusingcurrent source and to the digital voltage measuring circuit.
 12. Theapparatus of claim 11 further comprising a processor coupled to theswitch, the processor adapted to operate the switch to select which ofthe electrodes is coupled to the digital voltage measuring circuit,which of the electrodes is coupled to a measuring current source andwhich of the electrodes is coupled to the focusing current source. 13.The apparatus of claim 12 wherein the processor is adapted to selectrespective electrode connections by interpretation of command signalstransmitted to the apparatus from a control unit disposed at the Earth'ssurface.
 14. The apparatus of claim 12 wherein the processor is adaptedto select respective electrode connections based on measured voltagedrops across at least two of the electrodes.
 15. (canceled)
 16. Theapparatus of claim 1 further comprising a controller operatively coupledbetween the plurality of voltage measurement electrodes, the currentsource electrode and the digital voltage measuring circuit, thecontroller operable to connect selected ones of the plurality of voltagemeasurement electrodes and the current source electrode between thedigital voltage measuring circuit and the current source so as to makemeasurements of voltage drop representing at least one of selectedlateral depths of investigation and selected axial resolution. 17.(canceled)
 18. The apparatus of claim 16 wherein the controllercomprises programming for automatic operation of the first and secondswitches according to a predetermined sequence.
 19. The apparatus ofclaim 16 wherein the controller is adapted to detect commandstransmitted from the Earth's surface for reprogramming the operation ofthe first and second switches.
 20. (canceled)
 21. (canceled) 22.(canceled)
 23. (canceled)
 24. (canceled)
 25. (canceled)
 26. (canceled)27. (canceled)
 28. (canceled)
 29. (canceled)
 30. (canceled) 31.(canceled)
 32. (canceled)
 33. (canceled)
 34. (canceled)
 35. (canceled)36. (canceled)
 37. (canceled)
 38. An apparatus for measuring formationresistivity through a conductive pipe in a wellbore, comprising: a sondeadapted to be moved through the wellbore; a plurality of voltagemeasurement electrodes disposed on the sonde at spaced apart locations,the voltage electrodes adapted to make electrical contact with the pipe;at least one current source electrode disposed on the sonde, the currentsource electrode adapted to make contact with the pipe; a digitalvoltage measuring circuit controllably coupled to selected ones of thevoltage measurement electrodes; and at least one focusing current sourcecontrollable to maintain a selected voltage drop across a pair ofreference potential electrodes, the focusing current source electricallycoupled to selected electrodes on the sonde.
 39. The apparatus of claim38 wherein the digital voltage measuring circuit comprises of at least atwenty four bit resolution analog to digital converter.
 40. Theapparatus of claim 39 wherein the analog to digital converter has asampling rate of at least one thousand times a frequency of electricalcurrent used to energize the at least one current source electrode. 41.The apparatus of claim 38 further comprising a digitally synthesizedcurrent source coupled to the at least one current source electrode. 42.The apparatus of claim 41 wherein the current source is adapted togenerate switched direct current.
 43. The apparatus of claim 41 whereinthe current source is adapted to generate switched direct current havingless than a one hundred percent duty cycle.
 44. The apparatus of claim41 wherein the current source is adapted to generate alternating currenthaving a selected frequency and waveform.
 45. The apparatus of claim 41wherein the current source is adapted to generate a pseudo random binarysequence.
 46. The apparatus of claim 38 wherein the digital voltagemeasuring circuit is adapted to determine a direct current bias extanton the voltage measurement electrodes by operating substantiallycontinuously.
 47. The apparatus of claim 38 further comprising: at leastone focusing current source controllable to maintain a selected voltagedrop across a pair of reference potential electrodes, the focusingcurrent source electrically coupled to selected electrodes on the sonde;and a switch adapted to selectively connect selected ones of theelectrodes to the focusing current source and to the digital voltagemeasuring circuit.
 48. The apparatus of claim 47 further comprising aprocessor coupled to the switch, the processor adapted to operate theswitch to select which of the electrodes is coupled to the digitalvoltage measuring circuit, which of the electrodes is coupled to ameasuring current source and which of the electrodes is coupled to thefocusing current source.
 49. The apparatus of claim 48 wherein theprocessor is adapted to select respective electrode connections byinterpretation of command signals transmitted to the apparatus from acontrol unit disposed at the Earth's surface.
 50. The apparatus of claim48 wherein the processor is adapted to select respective electrodeconnections based on measured voltage drops across at least two of theelectrodes.
 51. The apparatus of claim 38 further comprising: a currentreturn electrode coupled to the pipe proximate the Earth's surface and acurrent return electrode disposed proximate the Earth's surface at aselected lateral distance from the current return electrode coupled tothe pipe, and a switch to select a return path for measuring currentfrom the current source electrode to the selected one of the electrodeat coupled to the top of the pipe and the electrode disposed at theselected lateral distance from the pipe.
 52. The apparatus of claim 38further comprising a controller operatively coupled between theplurality of voltage measurement electrodes, the current sourceelectrode and the digital voltage measuring circuit, the controlleroperable to connect selected ones of the plurality of voltagemeasurement electrodes and the current source electrode between thedigital voltage measuring circuit and the current source so as to makemeasurements of voltage drop representing at least one of selectedlateral depths of investigation and selected axial resolution.
 53. Theapparatus of claim 52 wherein the controller comprises programming forautomatic operation of the first and second switches according to apredetermined sequence.
 54. The apparatus of claim 52 wherein thecontroller is adapted to detect commands transmitted from the Earth'ssurface for reprogramming the operation of the first and secondswitches.
 55. An apparatus for measuring formation resistivity through aconductive pipe in a wellbore, comprising: a sonde adapted to be movedthrough the wellbore; a plurality of voltage measurement electrodesdisposed on the sonde at spaced apart locations, the voltage electrodesadapted to make electrical contact with the pipe; at least one currentsource electrode disposed on the sonde, the current source electrodeadapted to make contact with the pipe; a digital voltage measuringcircuit controllably coupled to selected ones of the voltage measurementelectrodes, the digital voltage measuring circuit including an analog todigital converter having at least twenty-four bit resolution and asampling rate of at least one thousand times a frequency of electricalcurrent used to energize the at least one current source electrode. 56.The apparatus of claim 55 further comprising at least one focusingcurrent source controllable to maintain a selected voltage drop across apair of reference potential electrodes, the focusing current sourceelectrically coupled to selected electrodes on the sonde.
 57. Theapparatus of claim 55 further comprising a digitally synthesized currentsource coupled to the at least one current source electrode.
 58. Theapparatus of claim 57 wherein the current source is adapted to generateswitched direct current.
 59. The apparatus of claim 57 wherein thecurrent source is adapted to generate switched direct current havingless than a one hundred percent duty cycle.
 60. The apparatus of claim57 wherein the current source is adapted to generate alternating currenthaving a selected frequency and waveform.
 61. The apparatus of claim 57wherein the current source is adapted to generate a pseudo random binarysequence.
 62. The apparatus of claim 55 wherein the digital voltagemeasuring circuit is adapted to determine a direct current bias extanton the voltage measurement electrodes by operating substantiallycontinuously.
 63. The apparatus of claim 55 further comprising: at leastone focusing current source controllable to maintain a selected voltagedrop across a pair of reference potential electrodes, the focusingcurrent source electrically coupled to selected electrodes on the sonde;and a switch adapted to selectively connect selected ones of theelectrodes to the focusing current source and to the digital voltagemeasuring circuit.
 64. The apparatus of claim 63 further comprising aprocessor coupled to the switch, the processor adapted to operate theswitch to select which of the electrodes is coupled to the digitalvoltage measuring circuit, which of the electrodes is coupled to ameasuring current source and which of the electrodes is coupled to thefocusing current source.
 65. The apparatus of claim 64 wherein theprocessor is adapted to select respective electrode connections byinterpretation of command signals transmitted to the apparatus from acontrol unit disposed at the Earth's surface.
 66. The apparatus of claim64 wherein the processor is adapted to select respective electrodeconnections based on measured voltage drops across at least two of theelectrodes.
 67. The apparatus of claim 55 further comprising: a currentreturn electrode coupled to the pipe proximate the Earth's surface and acurrent return electrode disposed proximate the Earth's surface at aselected lateral distance from the current return electrode coupled tothe pipe, and a switch to select a return path for measuring currentfrom the current source electrode to the selected one of the electrodeat coupled to the top of the pipe and the electrode disposed at theselected lateral distance from the pipe.
 68. The apparatus of claim 55further comprising a controller operatively coupled between theplurality of voltage measurement electrodes, the current sourceelectrode and the digital voltage measuring circuit, the controlleroperable to connect selected ones of the plurality of voltagemeasurement electrodes and the current source electrode between thedigital voltage measuring circuit and the current source so as to makemeasurements of voltage drop representing at least one of selectedlateral depths of investigation and selected axial resolution.
 69. Theapparatus of claim 68 wherein the controller comprises programming forautomatic operation of the first and second switches according to apredetermined sequence.
 70. The apparatus of claim 68 wherein thecontroller is adapted to detect commands transmitted from the Earth'ssurface for reprogramming the operation of the first and secondswitches.